1. Field of Invention
The present invention relates generally to noise cancellation systems and more particularly to an adaptive noise cancellation system capable of reducing received noise.
2. Status of the Prior Art
Considerable radio noise is generated by Personal Computers (PC's), as well as other portable computing devices. The noise created by these devices can interfere with the reception of signals by devices such as Wireless Wide Area Network Adapters thereby reducing the sensitivity of the adapter and hence the range to the base station.
The interference can be reduced by suppressing the noise at the source through improved design of the noise emitting electronic device. Alternatively, the noise can be reduced by choosing an antenna for the receiving device which isolates the antenna from the computer using distance (i.e., remote cable connection) or other means. However, these solutions have not been effective because of the reluctance of device manufacturers to increase product cost and a user's reluctance to use a remote cabled antenna.
A common problem with both PCMCIA and OEM wireless modules is that host generated noise can cause desense of the modem on one or more channels of the wireless data service. Desense refers to host generated Electro-Magnetic Interference (EMI) increasing the effective level of the noise floor and reducing the effective sensitivity of the receiver. Measurements have shown that desense in the laptop environment for the PCS band can be as high as 19 dB and for the 850 MHZ band can be as high as 30 dB.
The desense typically arises from digital clock noise generated by the computing device. The clock noise creates harmonics and other spectral components which lie within the bandwidth of the radio channel being used. If these spectral emissions occur within the channel being used for data communication, then problems of interference can occur. The emissions are strong enough to significantly degrade the input sensitivity of the receiver, even though their strength is low enough to meet regulatory emission requirements.
Most common current paths within an electronic device (such as a personal computer) consist of I/O cables, printed circuit board (PCB) signal traces, power supply cables, and power-to-ground loops. Each of these current paths can function as an antenna which radiates electric and magnetic fields. Interaction of these fields with other signals is EMI. The magnitude of the EMI is a function of several characteristics of the transmitted signal—such as frequency, duty cycle, and voltage swing (i.e., amplitude). Determining the role of transmitted signal characteristics is best analyzed in the frequency domain using a Fourier transformation. Any periodic function in the time domain f(t), may be represented by an infinite series of sines and cosines:f(t)=A0+A1 cos(ωt)+A2 cos(2ωt)+ . . . +An cos(nωt)B1 sin(0t)+B2 sin(2 ωt)+ . . . +Bn sin(nωt)  (1)whereω=2π/T  (2)andT=1/frequency.  (3)The magnitude of the coefficients An and Bn are determined from the duty cycle, edge rate, and magnitude of the digital signal.
If the signal is non-periodic (such as hardware with a microcontroller which references RAM, Flash, I/O devices, control lines, etc. in a time varying fashion), the Fourier Series representation of f(t) (i.e., Equation 1) would contain terms for a wide range of fundamental components such as fundamental frequencies and all of their harmonics.
In a typical PCMCIA or OEM installation, the signal spectrum near the logic boards would appear to be fairly wideband in nature and comprise a large number of individual spectral peaks whose amplitude would vary in time with the function being performed by the digital logic of the board. FIGS. 1 and 2 illustrate representative examples of time domain and frequency domain waveforms.
The frequency spectrum generated by the high clock speeds and sharp edges of clocks in modern digital devices can extend well into the GigaHertz region. As such, these signals may be within the allocated bandwidth of commercial communication services. As previously mentioned, these signals may be relatively low in amplitude to satisfy the requirements of regulatory emission levels. However, these signals are quite strong when compared to the Received Signal Strength Indication (RSSI) of wireless network transmissions. For example, the RSSI from a base station may be in the order of about −85 dBm, but the level of interference from nearby digitally generated noise may be in the order of −80 dBm. As is evident, a −5 dBm signal to noise ratio results in this example and would degrade the overall wireless network performance.
The present invention addresses the above-described interference problems associated with electronic devices by providing a system and method of adaptive noise cancellation. In this respect, the present invention provides a system which can adaptively cancel electronic device generated noise from nearby radiating electronic devices.